Mist ONLY around the horizon (cycles)
Cycles Volumetrics are rather slow and for this specific setup, a compositing solution may be more suited, however you can use the geometry information to limit the density of volumetrics.

In this example I created a large cube, encasing the scene, and made the Z information influence the scatter density. That way, the density is high near Z = 0 (the ground) and low as Z grows (the sky).